It is known in the field of printing that inkjet printing heads, often simply called heads, require periodic cleaning of printing nozzles, to remove buildup (solid sediments) on the nozzles, remove air bubbles, and maintain printing quality. Cleaning the printing head is a significant part of the inkjet printing process, for example in some industrial settings the printing head is cleaned as often as every two minutes. The frequency of cleaning depends on the specific application far which the printing head is being used. Simply stated, inkjet printers operate by expelling a small volume of ink from a plurality of nozzles though corresponding small orifices in an orifice plate held in proximity to a paper or other medium, also known as a substrate, upon which printing or marks are to be placed. The orifices are arranged in a fashion in the orifice plate such that the expulsion of droplets of ink from a selected number of nozzles relative to a particular position of the medium results in the production of a portion of a desired character or image. Controlled repositioning of the medium relative to the nozzles, followed by another expulsion of ink droplets, results in the creation of more segments of the desired character or image.
An orifice plate, as is generally known in the industry, is located on the printing side of the printing head, providing access for the nozzles to print, while also providing protection for the printing head, among other features. The outside or downward surface of the orifice plate is referred to as an orifice surface. Note that typically nozzles interface with the orifice surface via “cells”, with the jetting-end of each nozzle having a cell that surrounds the nozzle. The opening of the cell to the orifice surface provides an orifice. Jetted ink from each nozzle exits the orifice for printing.
During periodic cleaning and after purging, preferably the orifice surface is cleaned, known as wiping, to remove buildup, purged liquid, and enable proper jetting of the printing liquid from the nozzles (via the orifices). In order to preserve the smoothness and non-wetting (anti-wetting) characteristic of the orifice surface, care must be taken in performing wiping.
One conventional technique for wiping without contact to the orifice plate is vacuum wiping, where a vacuum head is moved across the orifice plate. The vacuum head does not contact the orifice plate but is sufficiently close to allow the vacuum, also known as suction, to remove the purged liquid from the orifice plate. As the vacuum head does not contact the orifice plate, there is suction from all sides of the vacuum head (not just from the direction of the orifice plate) resulting in low cleaning efficiency of the orifice plate. Disadvantages to conventional vacuum wiping include cost, printing speed, reliability, and quality of wiping.
Another challenge of wiping is when a mask, also called a cooling mask, is used with the printing head. A mask surrounds the printing head, providing protection for the printing head and functioning as an insulating shield, minimizing beat exchange between the printing head and a substrate. Protection includes protecting the printing head from excessive heat (or cold) from the medium (substrate) and from physical collision with objects on a printing tray. An example is printing metallization on a photovoltaic wafer, wherein the wafer is warmed before printing to 220 degrees Celsius. At least a portion of the mask is between the nozzles and the medium. The mask includes one or more slits corresponding to one or more nozzles. The slits are positioned and sized to allow jetted ink from the nozzles to pass through the mask (via the corresponding slit) to the printing medium. Typically and preferably, a row of nozzles on the orifice plate is offset only a small amount from the edge of the slit. Nozzles are offset only a small amount so the nozzles are located close to the edge of the slit in order to facilitate at least two goals. A first goal is to shield the nozzles from fumes emerging from the substrate. In this context of shielding, a small amount is in comparison to the size of the slit, with a typical offset being approximately 10% or less of the width of the slit. For example, when the slit width is 1 mm, the offset may be 100 μm or less. A second goal is to facilitate easier ink sucking under the mask during purge. In the context of easier ink sucking, a small amount is in comparison with a size of an orifice diameter, the size of a gap between the mask floor and orifice plate, the quality of non-wetting characteristics of the orifice, and the surface tension of the dispensed liquid. For example, with an orifice diameter of 20 μm, a gap of 150 μm, reasonable wetting characteristics, and reasonable ink surface tension, an offset of 150 μm or less has shown to be effective.
The use of a mask further reduces the efficiency of using vacuum cleaning to wipe the orifice plate. Refer to WIPO application IB11/051934 filed on May 2, 2011, which claims priority from U.S. provisional application 61/330,351 for additional information on masks.
When ink used for printing is a volatile liquid, the ink at a tip of a nozzle may lose a portion of the ink, with the remaining ingredients of the ink forming a semi-solid skin at the nozzle tip. The semi-solid skin, or buildup of solid sediments, can interfere with the jetting of ink from the nozzles, reducing the quality or even disabling jetting of ink from one or more nozzles. As the nozzle tips am aligned with orifices in an orifice plate, sediment buildup can also be on the orifices and/or orifice plate. In the context of this document, buildup on nozzles, orifices, and/or an orifice plate all present the same problem of sediment buildup. Because sediment can gradually build even during continuous printing, wiping the printing head/orifice plate should be done on a timely basis or in respect to a number of printing passes. Sediment buildup is a particular problem when printing pauses, or stops, for an extended period. During an extended period of non-printing, the liquid portion of ink that remains on, or in, the nozzles can evaporate, leaving behind sediment. When desiring to resume printing, time must first be spent wiping the printing head to clean the sediment from the nozzles.
There is therefore a need for a system for cleaning an orifice plate, with increased efficiency over conventional techniques, and preventing sediment buildup.